Fungi and yeast in particular provide useful systems for the production of many gene products, especially proteins, that have pharmacological, cosmetic, agricultural, industrial, or other commercial uses. Heterologous proteins may be synthesized, post-translationally modified, folded, and secreted in the context of a eukaryotic host.
Recombinant genes are typically placed under the transcriptional control of heterologous gene regulatory elements. Such elements are generally selected to provide high levels of gene expression, whether constitutive or inducible/repressible. However, constitutive gene expression provides neither controlled expression, temporally or otherwise, nor means for differentially expressing the recombinant protein of interest relative to undesired background gene expression.
Unfortunately, presently available regulated expression systems provide either inferior expression or limit host cells or expression conditions. For example, the strongest glycolytic promoters are induced upon the addition of glucose. Glucose however, favors the synthesis of most cellular proteins so these promoters do not permit differential expression of the target protein. Some phosphate-regulated promoters are available, but these are relatively weak promoters. Galactose-regulated promoters require that the culture be grown in galactose; a nutrient that will not sustain the growth of many yeast strains.
What is needed is a strong, inducible eukaryotic gene expression system, broadly adaptable to conventional expression hosts and capable of differentially expressing the target protein.